1. Field
The invention concerns improved methods and apparatus for hydrodischarging and hydrocharging substrates and articles to produce enhanced ability to avoid attraction of contaminants or improved capability to removing contaminants from fluids. The field of the invention relates to electret substrates and of filtration media. Additionally, the inventions relate to controlling electrostatic charge on substrates and cleaning substrates. The invention concerns the modification of electrical charge properties of substrate. The invention concerns in one form electret enhanced filter media made of substrate, and fibers such as blown microfibers. The invention concerns improved methods of making electret substrates and articles for removing particulates and mists from gas streams. In another form the invention concerns the removal of electric charges or neutralization of charge on or within substrates.
2. Background Information
The addition of electric charge to a substrate is quite useful. It is known that substrates including polymer materials may be semi-permanently electrically charged, or for brevity, charged. When charged, such polymers are known as “electrets”. Electrets have significant commercial value. For instance, the electric field produced by the electret can be used to attract other materials, such as dust particles. This attractive or “inductive” property exhibited by electrets substrates enables filters to be constructed having the ability to capture sub-micron particles when the pore sizes are many times larger.
The removal of electric charge from a substrate is also quite useful. Often it is required in the manufacture of substrate intermediates for many products to prevent dust contamination. Objects, including humans, very often acquire a sizable electrostatic charge which may have a magnitude of several thousand volts or more. Charging of non-conductive objects may be caused in many ways including frictional contact. Induction and discharges from other objects may impart charge to ungrounded conductors. Sizable charge accumulations can be highly undesirable for a number of reasons in the processing of dielectric materials and semiconductors. Sudden discharges even when not harmful are distinctly unpleasant to people. Electrostatic charge can also interfere with the operation of electrical devices including integrated circuits. Very important is that charge also attracts contamination.
The substrates include for example plastic films, paper, nonwovens, fabric materials, dielectric materials, and nonconductive materials. These are commonly used as the base of construction for a wide variety of greatly differing products. Some examples include photosensitized film, photographic print paper, magnetic recording tapes, adhesive tapes, pressure-sensitive paper, packaging materials, signage, filters, wrapping materials, electronic substrates, optical films, and cleaning products.
Prior art often requires the use of complex or hazardous processes for neutralization of static charges on a web or substrate, for the electrostatic modification of substrates, and for the production of charge species on or in the substrate. All of the known methods have various limitations and problems which restrict their utility or economics. These are discussed delineated in the following review.
Hydrocharging for the Production of Electrets
Electrets are dielectric objects that exhibit a lasting electric charge or a charge that is at least quasi-permanent. The charged nature of the electret enhances the electret's ability to attract and retain aerosol particles, and contaminants such as dust, dirt, and fibers that are present in fluids. Electrets have been found to be useful in a variety of applications including air filters, furnace filters, respiratory filters, face masks, and electro-acoustic devices, headphones, and electrostatic recorders. Commonly, nonwoven or fabric substrates are used in filtration. Electrets are especially useful for collecting micron and submicron size particles or aerosols on or within media whose pores or void spaces are much larger.
Nonwoven fibrous filter webs have been made from polyolefins using melt-blowing apparatus of the type described in Wente, Van A., Superfine Thermoplastic Fibers, Industrial and Engineering Chemistry, v. 48, n. 8, pp.—1342-1346 (August 1956). Such melt-blown microfiber webs are in widespread use for filtering contaminants, e.g., as face masks, furnace filters, and respirators. Melt-blown microfibers are commonly referred to as blown microfibers.
It is known that the filtration qualities of a blown microfiber web can be improved by a factor of two or more by making it an electret substrate. In one method the melt-blown fibers are bombarded with electrically charged particles such as electrons or ions as they issue from the die orifices. Similarly, the web can be made an electret by exposure to an electric corona after it is collected. While blown polypropylene microfibers are especially useful, other polymers may also be used including, for example, polyolefins, polycarbonates and polyhalocarbons. Most commercially useful are those materials that have appropriate volume resistivities under expected environmental conditions.
Filters for removing particulate contaminants from air and fluids are also made from other types of media. Examples include spunbond nonwoven media, woven fabric media, structured films, porous films and fibrillated films. U.S. Pat. No. RE 32,171 to van Turnhout teaches that electret filtration enhancement can be provided by electrostatically charging a film before it is fibrillated. However, the method uses high voltage charging which adds manufacturing expense and electrical hazards.
Hydrocharging is generally considered a process for preparing electret items and substrates without the use of high voltages, but it presently has deficiencies. It employs a liquid as a charging agent. Angadjivand et al. in U.S. Pat. No. 5,496,507 teach impinging water upon a nonwoven web with jets or droplets, then drying the web to create an electret media. While the technique has been described by some as a method of triboelectic charging, the details of the electrification process are not fully explained.
Hereinafter “hydrocharging” refers to the contacting of a substrate with a liquid to create an electret. While the Angadjivand et al. hydrocharging process develops filtration enhancing properties, the degree of treatment is deficient and pre-charging the web by corona charging prior to hydrotreatment is necessary for the best results. This teaching still requires the capital investment and the operating cost of corona generating devices along with their high voltages. More effective processes are sought.
Further improvements in hydrocharging are taught by Eitzman et al. in U.S. Pat. Nos. 6,406,657 and 6,824,718. They teach the multiple steps of wetting with a wetting liquid, followed by saturation with an aqueous polar liquid, followed by drying. Wetting liquids with surface tensions below the surface energy of the fiber are taught for the wetting step. But even so, air is trapped in the web and the use of mechanical means to help in the removal of trapped gas is taught. The use of an aqueous polar saturation liquid with a surface tension higher than and preferably 10 dynes per centimeter higher than the surface energy of the fibrous web is required after the wetting step. Careful formulation and control of both wetting liquid and the aqueous polar saturation liquid formulations are required for the process to function. The organic solvent isopropyl alcohol is the wetting agent of the examples. As such, the process uses an expensive and hazardous chemical to achieve results.
Eitzman et al. in U.S. Pat. No. 6,454,986 teach the use of flammable polar organic solvents by themselves to create electret media. This still has the disadvantages of high costs, and explosion and fire hazards associated with the solvent. Surface tensions of 10 dynes per centimeter higher than the surface energy of the fibrous web are preferred. Wetting is not complete. A partial remedy of this problem using various mechanical agitation means are taught to aid wetting. These add cost and complexity and have limited success.
Horiguchi and Takeda in USPTO Publication Number 20040023577 expound a method of hydrocharging by sucking water through a fibrous substrate followed by drying. The efficiency of the process is deficient. They note the electret quality is improved by repeating the suction process multiple times before drying. The teaching recommends use of a wetting solvent in the water. Elimination of repeating steps is desired to reduce process cost and complexity. Elimination of cost and hazard of the wetting solvent would be a cost saving. Wetting problems are still present even with the wetting solvent.
The process of depositing liquid from a vapor onto a dielectric article prior to drying is taught in U.S. Pat. No. 6,743,464 to Insley et al. Here, the method utilizes the complicated process of first creating a controlled environment using a closed vessel containing a liquid and a gas phase, and second, manipulating a thermodynamic state function such as pressure to cause molecules of the liquid component present in the gas to condense as liquid drops upon the article. This condensed liquid on the article is then dried. The gas phase contains noncondensible air diluting the molecules of the liquid species present in the gas phase. The complexity of this method is a disadvantage. Additionally, condensation must take place, no means of removing trapped air in the article is provided and no method of treatment of a continuous web is taught. The method does not teach a means of complete and total wetting of substantially all surfaces in pits, voids, pores and internal spaces of the substrate.
Improved and simplified hydrocharging methods are needed that do not use costly or hazardous materials, do not require high voltage electrical auxiliary treatment and do not require special chemical formulations. Methods that do not use polluting chemicals are sought. Methods that act with the improved efficiency are needed. Methods that produce improved contacting of all surfaces in pits, voids, pores and internal spaces of the substrate and improved methods for treating continuous webs are needed. Methods that remove greater amounts of air from the surfaces and pores of the media would result in the more complete treatment of all the potentially functional surfaces of the substrates.
Electret Surface Contamination
An additional problem for electret media is contamination. Oily contamination from a gas stream being filtered is noted as being highly detrimental to the efficacy of filtration products in U.S. Pat. Nos. 5,411,576 and 6,802,315. Much effort has been devoted to producing oil mist tolerant electret media. However, other sources of contamination have been unrecognized. One is the problem of surface contamination of the electret media during manufacturing, which has not been recognized and addressed.
The processes necessary for the forming of the substrates, and the processing and handling of the substrates may contaminate the active surfaces. In the production of fibers and films, high temperatures are used. The materials are extruded in a molten state. Thin films, very fine fibers, and especially melt blown and spun bond fibers are extruded from melts and are most easily produced when the melt viscosity is as low as possible. Low melt viscosity is achieved at extremely high temperatures. Often these temperatures exceed the thermal stability of the materials extruded. At high processing temperatures thermal degradation forms oil-like low molecular weight contaminants. The oil-like liquid degradation products produced commonly cover the surfaces of fume hoods over these melt process lines. Smoke and fumes are often observed rising from molten polymers being extruded, milled, or melt spun. These then may condense on, or be adsorbed by the functional surfaces of the electret substrates or precursor substrates produced.
The process equipment used for transporting, forming, collecting and extruding substrate materials uses hydraulic and lubricating oils along with other liquids which are electret contaminating species. These will often contaminate filter media. Oil and decomposition contaminants tend to spread and cover the active surfaces of many common polymers used for filters. This is especially true of the polyolefin polymers. Such contamination can diminish either the initial or long term performance of electrets.
Here it has been found that any liquids that spread on the substrate or substrate functional surfaces are detrimental. Still other harmful contaminants include species that modify the wetting characteristics of liquid, collected on the electret and cause liquid to spread on the substrate surfaces.
The problem of counteracting contamination from the forming methods remains unsolved and generally unrecognized.
Electret Substrates for Filtering Liquid Mists
The filtration properties of nonwoven and fabric polymeric fibrous webs can be improved by transforming the web into an electret. Electrets are effective in enhancing initial liquid aerosol particle capture in filters. But with time or aging, liquid aerosols tend to cause electret filters to lose their enhanced filtering efficiency. This subsection deals with the art of preparing improved aerosol filter media.
Numerous methods have been developed to compensate for loss of filtering efficiency with time or aging in the presence of mists. One method includes increasing the amount of the nonwoven polymeric web in the electret filter by adding layers of web or increasing the thickness of the electret filter. The additional web, however, increases the breathing resistance of the filter, adds weight and bulk to the filter, and increases the cost of the filter. Another method for improving an electret filter's resistance to oily aerosols includes forming the electret filter from resins that include melt processable fluorochemical additives, such as fluorochemical oxazolidinones, fluorochemical piperazines, and perfluorinated alkanes.
A method of improving the performance of an electret is taught by Jones, et al. in U.S. Pat. No. 5,472,482. It teaches placing a performance enhancing fluorochemical additive into the polymer melt, extruding the blend in the form of a microfiber web, and then charging the web. These additives are referred to as “charge additives.” U.S. Pat. No. 5,645,627 also teaches the use of charge additives. The charge additives can increase the level of charge on the electret and can improve the filtering performance of the electret. The charge additives have been found by experimentation. Charge additives within the mass of polymer must be melt processable, i.e., suffer substantially no degradation under the melt processing conditions used to form microfibers of a nonwoven web or the fibers and films of electret substrates. This limits possible candidate additives.
The improved performance in the additive patents is only demonstrated with one, and only one, aerosol liquid mist. This unduly limits useful candidate additives. Tests are only made with dioctylphthalate (DOP) in air at standard conditions. The test is hereinafter referred to as the “DOP challenge”. No information is provided for filtering other liquid aerosols in other gases and at other conditions.
In U.S. Pat. No. 5,935,303, an improved filter is taught which uses a resinous material containing a perfluoroalkyl acrylate adhering to the fibrous substrate in a filter. This improvement is again only tested against a DOP challenge. No information is provided for filtering other aerosols and mists.
In U.S. Pat. No. 6,213,122, a method of making an electret with improved DOP filtering performance by including the step surface fluorination is taught. Fluorination is a costly and sometimes a very hazardous step. Again, the electret is only tested against the DOP challenge.
In U.S. Pat. No. 6,238,466, an electret article with improved oily mist performance is disclosed where the formulation includes a charge additive and passes a thermally stimulated discharge current (TSDC) test. However, again the electret is only tested against the DOP challenge.
U.S. Pat. No. 6,214,094 teaches electret articles using charge additives to produce improved DOP challenge performance. Here too, the electret is only tested against the DOP challenge.
U.S. Pat. No. 6,802,315 discloses electret articles using vapor condensed coatings with fluorine contents that give improved results. Here too, the electret is only tested against the DOP challenge.
U.S. Pat. No. 6,237,595 teaches electret DOP filtering performance may be predicted by measuring extractable hydrocarbons.
U.S. Pat. No. 6,261,342 teaches electret DOP filtering performance may be predicted by a thermally stimulated discharge current (TSDC) spectrum.
USPTO Publication Number 20030054716 teaches treating porous substrates with a solvent composition which includes a charge additive for enhancing performance for the DOP challenge. However, these solvents are expensive and usually dangerous.
None of the prior art teaches how to improve the performance of electrets for liquid mists other than DOP. None of the prior art teaches how to improve the performance of an electret filtration process for a specific liquid in gas mist challenge other than DOP. The DOP testing criteria limits the number and type of chemicals that may be used as charge additives and is therefore, unduly restrictive.
A method of making an electret for a target liquid contaminant in a target gas at target conditions is needed.
Electret Improvement by Surface Treatments
The filtration properties of electret webs can be improved by applying surface modifying chemicals to the surfaces that interface with the fluid being filtered. Chou et al. in U.S. Patent Application Publication 20030054716 teaches the swelling of an electret filter substrate polymer with a solution containing a filtration enhancing additive. Upon evaporation of the solvent, additives are left behind within the polymer and on its surface. While this is an efficacious process, it has the disadvantage of requiring the use of costly and often dangerous solvents along with the costly step of drying the solvent from the substrate. Additionally, using solvents may be environmentally harmful. Evaporation and loss of the solvents by drying consumes these expensive materials.
In U.S. Pat. No. 6,802,315, it is taught to produce electret media using vapor condensed coatings on the surfaces of the fibers. Improved filtration properties are achieved. Here, the range of surface coating compositions is limited to coating precursor monomers that may be evaporated. The process can be costly.
Other patents teach chemical modification with reactive plasma or gaseous reactants. In U.S. Pat. No. 6,660,210, surface fluorination is used to produce modified and improved electret performance. This process limits surface modification to only fluorination and substrates that may be fluorinated. Fluorination usually involves hazardous chemicals and expensive equipment.
New and more flexible methods of applying surface modifying chemicals and filtration modifying species are needed to overcome the limitations of known methods.
Electrostatic Neutralization and Control
This subsection of art deals with the field of reducing and neutralizing electrostatic charge on dielectric and other materials.
One may speak of surface modification in terms of the energy expended per unit area of modified surface. On the low energy extreme, it is desirable to neutralize static charge on substrates. Surface and volume charge on a dielectric material can exist as a net or monopole charge and/or as dipoles of charge in isolated regions. Accumulation of such charge can occur in a wide number of circumstances and with a wide range of dielectric material forms such as thin films, webs, sheets, fibers and threads. These may be made of paper, plastic, textiles, etc. Static charge is generally always present to some degree and nearly impossible to avoid. In sheet or web transporting, it is well known that electrical charges can build up on non-conductive materials. In industry the presence of charges is detrimental in at least three different ways. They may create safety hazard problems. They may interfere with product or process functions, or they may contribute to surface contamination.
Regardless of the form of the material, the accumulation of net charge on a dielectric material presents potential electrostatic hazards that often need to be eliminated or significantly reduced. For example, reduction or elimination of net charge is important during operation in hazardous environments, such as with a charged web moving in proximity to explosive vapors. Charge densities may spontaneously generate electrostatic discharges and ignite the flammable vapors. Electric discharges from substrates especially at web winding stations can produce arcing discharges that are hazardous to operating personnel. Neutralizing charges on sheets or webs is also necessary to facilitate trouble free passage and directing of web or sheets through processing equipment, especially in the stacking and collating processes. This is often referred to as the elimination of static cling.
Control of substrate surface charge is important in the process of coating a continuously traveling web support with compositions such as photographic emulsions, magnetic coating compositions, functional coatings for liquid crystal display screens, flexible electronic substrates, and many others. Particulate and mist contaminants are attracted to and are held on substrates by charges. It is important to minimize this in the production of photographic light-sensitive products, printing plates, pressure-sensitive copying papers, light emitting diodes, electronic substrate precursors, light display screens, optical products, etc. Clean substrates are essential in the manufacture of electronic and optical surfaces. Contaminants are a prime source of product defects even for those manufactured in so-called clean rooms. In the manufacture of many of today's sophisticated new products with optical or electronic functionality, the presence of even very small differences in charge or uniformity of charge may create defects in the deposition of materials or the localized functionality of the product. Such situations are not unique to those products where a plastic or paper material is employed, but similarly apply to those products where a glass plate, semiconductor wafer or ceramic substrate is employed. An example is a glass base plate for a liquid crystal display or the like. The need for improved neutralization of charge is growing ever more demanding and important.
In general, handling webs of dielectric materials generates static electric charge in the material. It is well known and referred to as the triboelectric effect. When two members are moved relative to each other, the frictional contact between the surfaces generates a static electric charge on the surfaces. The separation of two surfaces in intimate contact will also generate charges. For example, the simple process of web movement around a roller without slippage will generate electrostatic charging. In web processing industries, static charge causes difficulties as described above. The processes of roll formation, slitting, coating, functionalizing or laminating are troubled by static charge.
Troublesome electrostatic charges on charge retaining materials may be grouped into two categories. One category is that of polarization charges or dipoles, and the other is free surface charges. Polarization charges are bound to a definite site in a solid, whereas free surface charges are not. Free surface charges on a moving web are frequently reduced by a grounded brush-like device such as that described in U.S. Pat. No. 3,757,164 to Binkowski.
Polarization charges in a web are commonly controlled by subjecting the web to a corona-generated electrostatic field having a particular magnitude and polarity. It is often necessary to deal with both categories of charges. Often with polarization charges or dipoles, there are combinations of both positive and negative charges.
Much effort over the last fifty years has been expended in providing clouds of positive and negative ions which are attracted to the respective oppositely charged areas on a substrate. U.S. Pat. No. 983,536 discloses a static neutralizing device wherein an insulated conductor with large surface area is positioned over a moving web of dielectric material and is impressed with a high AC voltage.
U.S. Pat. No. 3,364,726 discloses a static neutralizing device wherein an insulated fine wire electrode is impressed with AC voltages at various frequencies ranging from 300 to 2000 Hertz, depending upon the speed of the web material to be neutralized. The fine wire electrode is required to be positioned very near to the moving web, and there is also a requirement for a conductive metallic ground bar to be positioned nearby. This type of device creates a cloud of both positively and negatively charge species which are attracted to oppositely charge regions on the web. AC ionizers leave a frequency signature of charge on a moving substrate that can cause non-uniformities and are incapable of reducing substrate charge to near zero values.
Kisler in U.S. Pat. No. 4,363,070 teaches the use of a brush-like device of conductive filaments powered by an AC potential to provide the charged active species. Many wire and needle devices and improvements to them, the methods of using them, and the methods of controlling them have been invented. These include the teachings of Halleck in U.S. Pat. No. 4,729,057, Durkin in U.S. Pat. No. 5,432,454, Pitel et al. in U.S. Pat. No. 5,930,105, Wright et al. in U.S. Pat. No. 5,017,876, Steinman et al. in U.S. Pat. No. 4,951,172, Blitshteyn in U.S. Pat. No. 4,872,083, Halleck in U.S. Pat. No. 4,729,057, and Simons in U.S. Pat. No. 4,216,518. However, brush dischargers are only effective when the charge density is high, and they only reduce charge levels from high to lower values. Residual charge remains on the substrate.
All known methods of neutralizing charges suffer from additional defects. If both positive and negative charges are present in the same charge-retaining substrate and if positive and negative charges are to be neutralized by having their charge levels reduced to zero, then the application of a DC-type electrostatic field having either a positive or a negative polarity will not reduce the charge level to zero.
Any device employing a corona producing wire will suffer from wire contamination and produce non-uniform treatment along the wire. Multiple needle devices become non-uniformly dirty with time and produce non-uniform treatment. Also, devices producing coronas may produce ozone gas which is a hazardous material. Conductive devices rubbing upon a substrate may scratch and produce defective products. More importantly, while many of these devices are effective in reducing electric field strengths from tens of thousands of volts per centimeter to thousands, this is simply not sufficient for today's products. Improved performance is desired with field intensities reduced to near zero or to below 10 volts per millimeter and below 1 volt per millimeter.
Improved static removal methods are needed to overcome the deficiencies of the known art.
Substrate Cleaning
Particulate contamination of substrates is responsible for huge volumes of scrap product, especially in the photographic, electronic and optical industries. Static charge on the substrate attracts particles and holds them tenaciously to the surface. Removal of static charge and web cleaning are both essential for defect reduction in manufacturing.
Takahashi et al. in U.S. Pat. No. 6,176,245 teach a series of apparatus for cleaning and charge removal: a first apparatus for the application and a second for the partial removal of a cleaning organic solvent mixture. The second generates static electric charge during the removal step. This charging is diminished by the immediate application of an under coat solution containing an organic solvent and resin composition which when dried, produces a coating for some functional purpose such as a protective coating, a magnetic coating, etc. This operation uses expensive and hazardous solvents, two separate application devices, and leaves a functional coating on the substrate.
Many methods for removing particles from the surface of a web are known including air knives, suction cleaning systems, wipes, and particle transfer rollers. In non-contact web cleaners, air at high velocity is passed over the surface of the web to remove particles. It is also common to attempt neutralization of web surface charge prior to cleaning to reduce the attractive forces between particles and the web. Unfortunately, complete neutralization is difficult and not achieved. U.S. Pat. Nos. 2,980,933, 4,213,167, 5,421,901 and 4,454,621 disclose devices for employing air streams and modification of the electrostatic charge on the web and/or particles. The results are often not satisfactory, and the use of tacky surface, contact cleaning systems have attempted to produce improvements. Tacky contacting surfaces produce detrimental static charging by contact. Non-contacting methods are less effective than the contact methods.
A particle removal roller typically has an adhesive or tacky surface to which particles from the web surface adhere upon contact. As the particles accumulate on the roll, the roll becomes contaminated and must be cleaned periodically to restore and renew its effectiveness. Contacting the web with a roll or mechanical wipes produces static charging which is counter productive as this charging will attract more particles from the environment. U.S. Pat. No. 5,930,857 teaches improvements to the contact roll method as do many other patents noted in its prior art description. Still, the contact cleaning apparatus may also scratch the substrate surface further degrading quality.
Ernst et al. in U.S. Pat. No. 5,425,813 teach the wet cleaning of a cleaning contact roll while it is disengaged from performing the web cleaning function. Here, a cleaning solution of alcohol and water is used to wet fabric wipes which clean the contact roll. The contact roll requires drying before reengagement with the web. This design requires at least two complete systems to provide continuous web cleaning of a running web.
Improved cleaning methods are needed to overcome the deficiencies of the known art.